Silica/silicate deposition in aqueous systems, for example boilers, cooling towers and systems containing hypersaline geothermal brines, is a continuing problem. Traditionally, deposition has been controlled by softening the makeup water to the system being treated, by blowdown, or by both. If deposition occurs, mechanical removal or washing with ammonium fluoride or hydrofluoric acid is generally the method of control. Obviously, mechanical or chemical cleaning causes down time and increased energy and labor costs.
pH affects the ionization of silanol groups and, therefore, affects the polymerization rate. It is believed that silica first forms, followed by the formation of three dimensional networks. Eventually, colloidal particles grow through condensation. At pH 7, nuclei formation and particle growth is very rapid. The pH of cooling water is generally 6.0 to 8.5 and the water temperature is generally about 30.degree. to 70.degree. C. The pH of geothermal brines is generally 4.0 to 6.0 and the brine temperature is generally about 100.degree. to 210.degree. C.
It is known to use cationic polymers or cationic surfactants as silica scale inhibitors in hypersaline geothermal brines (Harrar, J. E. et al, "Final Report on Tests of Proprietary Chemical Additives as Anti-scalants for Hypersaline Geothermal Brine", January 1980, Lawrence Livermore Laboratory, Harrar, J. E., et al, "On-Line Tests of Organic Additives for the Inhibition of the Precipitation of Silica from Hypersaline Geothermal Brine IV, Final Tests of Candidate Additives", February 1980. Lawrence Livermore Laboratories; and Harrar, J. E. et al, "Studies of Scale Formation and Scale Inhibitors at the Salton Sea Geothermal Field", Corrosion/80. Paper No. 225, International Corrosion Forum, devoted exclusively to the Protection and Performance of Materials, Mar. 3-7, 1980. Chicago, Ill.).
Also, copending Calgon patent application Ser. No. 290,798 discloses the use of phosphonates such as hexamethylene diamine tetra (methylene phosphonic acid) and diethylene triamine penta (methylene phosphonic acid) and anionic polymers to control silica deposition, and copending Calgon patent application Ser. No. 190,621, discloses the use of phosphonates such as 2-phosphonobutane tricarboxylic acid-1,2,4 and maleic acid/dimethyldiallyl ammonium chloride-type polymers to control silica deposition.
U.S. Pat. No. 3,928,196 discloses the use of copolymers of 2-acrylamido-2-methylpropylsulfonic acid and acrylic acid as scale inhibitors.
U.S. Pat. No. 4,640,793 discloses the use of admixtures containing carboxylic acid/sulfonic acid polymers and phosphonates as scale and corrosion inhibitors.
U.S. Pat. No. 4,618,448 discloses the use of polymers comprising an unsaturated carboxylic acid, an unsaturated sulfonic acid and an unsaturated polyalkylene oxide as scale inhibitors.
Japanese No. 57-084794 discloses the use of copolymers of acrylic acid and allyl polyethylene glycol as scale inhibitors.
European patent application 84301450.7 discloses carboxylic acid/sulfonic acid copolymers in combination with organic phosphonates as scale inhibitors.
U.S. Pat. No. 4,510,059 discloses the use of carboxylic functional polyampholytes to reduce silica deposits in aqueous systems.
U.S. Pat. No. 4,432,879 discloses the use of 2-phosphonobutane-1,2,4-tricarboxylic acid and methacrylic acid/2-acrylamido-2-methylpropyl sulfonic acid polymers to disperse solid matter such as clay including China Clay (Al.sub.2 O.sub.3.2H.sub.2 O.2SiO.sub.2) in aqueous systems. Threshold inhibition of silica/silicates is not disclosed or suggested.
U.S. Pat. No. 4,532,047 discloses a method of inhibiting amorphous silica scale formation using polypolar organic compounds and borate ion sources.
U.S. Pat. No. 4,584,104 discloses a method of inhibiting amorphous silica scale formation using a source of orthoborate ions.